In one aspect, this invention relates to an improved shell and tube heat exchanger. In another aspect, the invention relates to apparatus and methods for reducing the pressure drop in the flow of shell side fluid in a shell and tube heat exchanger.
Heat transfer is an important part of any process. As is well known, an indirect transfer of heat from one medium to another is usually accomplished by the use of heat exchangers, of which there are many types. For example, there are double pipe, shell and tube, plate heat exchangers and others. Indeed, the art of heat exchanger design is developed to a very high degree; however, there is still room for improvement in a number of areas, such as reducing pressure drop, increasing overall heat transfer coefficients, reducing fouling and in heat exchangers utilizing a tube bundle, such as the shell and tube heat exchangers, in improving the flow of the medium through the shell in contact with the tube bundle. In shell-and-tube heat exchanger designs, it is frequently advantageous to utilize "vapor belts" or annular distributors to reduce shellside inlet and exit pressure losses, reduce impingement velocities, and improve shellside fluid distribution. In Standards of Tubular Exchanger Manufacturers Association, 6th Edition, 1978, the following shell side impingement protection requirements are set forth in Section 5, page 29:
"An impingement plate, or other means to protect the tube bundle against impinging fluids, shall be provided when entrance line values of p.nu..sup.2 exceed the following: non-corrosive, nonabrasive, single-phase fluids 1500; all other liquids, including a liquid at its boiling point, 500. For all other gases and vapors, including all nominally saturated vapors, and for liquid vapor mixtures, impingement protection is required. .nu. is the linear velocity of the fluid in feet per second and .rho. is its density in pounds per cubic foot. A properly designed diffuser may be used to reduce line velocities at shell entrance."
An annular distributor is conventionally designed such that the ratios of nozzle-to-annulus flow area and annulus-to-slot flow area provide a recovery of static pressure by virtue of reduced momentum with passage through the nozzle, annulus, and shell slots. The exact magnitudes of these area ratios required to fulfil this criterion are not precisely predictable. If these area ratios are incorrectly specified by the designer, the pressure recovery through the annular distributor may be less than optimal and possibly result in a negative pressure recovery (i.e., positive pressure loss). It is thus desirable to provide apparatus and methods for adjusting such flow areas in tube and shell heat exchangers fitted with annular distributors, so that pressure drop can be minimized, particularly in the areas between nozzles and the shell interior, and flow through the shell and over the tube bundle optimized.